This invention relates to an arrangement for the separation of a suspension or emulsion from liquids by flotation. The arrangement comprises a flotation vessel with a mantle, or a side wall, means for supplying a liquid to be treated to the vessel, means for the removal of the treated liquid, and means for the removal of the foam which has been separated therefrom.
The flotation process for separating a suspension from liquids is frequently more efficient than processes based upon the gravitational separation of the suspension by sedimentation, and possibly also more efficient than processes using fluid filtration. Particularly in cases in which the separated suspension has a tendency for spontaneous flotation, flotation methods are also advantageous economically. For instance, that is the case in which waste waters containing fats in the meat industry have to be cleaned, and when waste waters containing oil from the food industry, and waste waters from petrochemical plants and the like have to be cleaned.
A wider application of separation of suspensions from liquids by flotations is permitted by the technology of the addition of flotation agents to the liquid to be cleaned. However, the separation of similar waste waters by flotation, frequently from large quantities of water, presents some difficulties in arrangements which are actually now being used.
The types of arrangements now being actually mostly used for the flotation of large quantities of liquids have the shape of rectangular vessels which are mostly made of concrete. The supply and distribution of air in the flotation apparatus uses a pressure saturation of a part of the treated liquid by air (at a pressure of 0.3 to 0.5 MPa) which is accomplished outside the flotation vessel. The liquid saturated with air is thereafter, after pressure reduction, mixed with the main stream of treated liquid, and the mixture of both parts of the liquid is introduced into a flotation vessel where the surplus of air is separated in the form of small air bubbles sticking to the surface of the suspension in the liquid. The suspension particles with separated air particles are thereafter taken along to the surface of the flotation vessel where they are mechanically removed by rakes or by sucking-off devices. A part of the suspension which drops to the bottom of the flotation vessel also has to be removed. The method of removal by rakes is also here frequently used.
Flotation apparatus in the shape of vertical cylindrical vessels have also been recently used. The supply of the treated liquid and the supply of a part of the treated liquid saturated under pressure with air is here, after pressure reduction, introduced into a central distributing space in the lower part of the flotation reactor. The removal of the flotated suspension on the surface, and the removal of a part of the suspension sedimented on the bottom of the flotation apparatus is accomplished in a manner similar to that employed with rectangular flotation apparatus, that is, by rakes or sucking-off means. The raking and sucking-off means is, in such case, fixed on a rotating arm turning about a central axis.
In large vessels, these mechanical means for removal of the flotated and sedimented suspension are relatively complicated, and also complicate the construction of the vessel proper of the flotation apparatus. Particularly with flotation arrangements in which, in order to obtain the required efficiency of separation of colloidal suspensions, an adjustment of the pH value by acids up to a value is used, and the mantle or wall of the flotation apparatus has to be resistant to acids, the provision of the arrangement with the above-mentioned means for removal of sludge substantially increases the total cost of the arrangement.